Multiply.
Explanation: When a number is next to a variable, it means they are multiplied. So, $({-5}{z^5})({6}{z^6})$ is the same as $({-5})({z^5})({6})({z^6})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(-5)(6)}{(z^5)(z^6)}$ Then simplify, and we're done! ${-30}{z^{11}}$